1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image in electrophotography, electrostatic recording, electrostatic printing, and the like. Specifically, the present invention relates to a toner for use in electrophotographic apparatuses such as copiers, laser printers, and facsimiles.
2. Discussion of the Background
In a typical electrophotographic image forming process, an electrostatic latent image is formed on a photoreceptor containing a photoconductive material and developed with a developer to form a visible image. The visible image is transferred onto a recording medium such as paper, and fixed thereon by application of heat, pressure, solvent vapor, and the like.
Methods for developing an electrostatic latent image are broadly classified into liquid developing methods using a liquid developer in which a pigment or a dye is finely dispersed in an insulative organic liquid, and dry developing methods, such as a cascade method, a magnetic brush method, or a powder cloud method, using a dry developer (hereinafter “toner”) in which a colorant, such as carbon black, is dispersed in a resin. The dry developing methods are becoming widely used recently.
On the other hand, as a method for fixing an image formed with a dry developer, i.e., a toner image, on a recording medium, a heat roller method is widely used from the viewpoint of energy efficiency. In recent attempts to reduce energy consumption in fixing, toners are required to be fixable at low temperatures. In other words, a smaller amount of energy is required when a toner image is fixed on a recording medium. The International Energy Agency (IEA) Demand-Side Management (DSM) program in 1999 involves a technology procurement project for next-generation copiers, and a requested specification is disclosed therein. Specifically, copiers with a printing speed of 30 cpm or more are required to have a warm-up time of 10 seconds or less and to consume energy in an amount of from 10 to 30 watts in the warm-up, which is a drastic energy-saving requirement compared to conventional copiers. To respond to the requirement, one possible approach involves reducing heat capacity of a fixing member such as a heat roller, so that temperature response of a toner is improved. However, this approach is insufficient to respond to the requirement.
To minimize the warm-up time, it is necessary that toners are fixable at low temperatures. To respond to such a requirement, toners using polyester resins, which are fixable at lower temperatures and have better thermostable preservability than conventionally-used styrene-acrylic resins, are disclosed in Unexamined Japanese Patent Applications Publications Nos. (hereinafter “JP-A”) 60-90344, 64-15755, 02-82267, 03-229264, 03-41470, and 11-305486. On the other hand, JP-A 62-63940 discloses a toner including a binder resin including a non-polyolefin crystalline polymer so as to improve fixing ability at low temperatures (hereinafter “low-temperature fixability”), and Japanese Patent No. (hereinafter “JP”) 2931899 discloses a toner including a crystalline polyester. However, the molecular structure and molecular weight of these crystalline polymers are not optimized therein.
None of the above-described toners satisfy the required specification of the DSM program. Therefore, a technology for further improving low-temperature fixability is needed. One possible approach involves controlling thermal properties of a binder resin, such as reducing the glass transition temperature (Tg) and/or molecular weight thereof. However, too much reduction of the glass transition temperature causes deterioration of thermostable preservability. In addition, too much reduction of the molecular weight decreases the softening point, and thereby decreasing a minimum temperature at which hot offset occurs. The “hot offset” here refers to an undesirable phenomenon in that part of a fused toner image is adhered to the surface of a heat member, and re-transferred onto an undesired portion of a recording medium. Consequently, a desired toner cannot be obtained only by controlling thermal properties of binder resins.
Methods for manufacturing toner are broadly classified into pulverization methods and polymerization methods.
In a pulverization method, a thermoplastic resin, a colorant, a charge controlling agent, an offset inhibitor, and the like, are evenly melt-mixed, and the mixture is then subjected to pulverization and classification. The pulverization method is capable of manufacturing a toner having a certain level of desired properties. However, there is a drawback that materials usable for the pulverization method are limited. For example, the toner composition is required to be treatable by an economical pulverization and classification apparatus. Therefore, the toner composition needs to be brittle. However, a brittle toner composition tends to produce particles with a broad particle diameter distribution by pulverization. To produce a copy image having good resolution and gradation, fine particles having a particle diameter of 4 μm or less and coarse particles having a particle diameter of 15 μm or more need to be removed, resulting in low yield. Further, it is difficult to evenly disperse a colorant, a charge controlling agent, and the like agent, in a thermoplastic resin by the pulverization method. Therefore, fluidity, developability, and durability of the resultant toner and image quality of the resultant image may deteriorate.
In attempting to solve the above-described problems of the pulverization method, polymerization methods have been proposed. For example, suspension polymerization methods and emulsion aggregation methods, such as the method disclosed in JP 2537503, are known. However, toners including a polyester resin, which may have good fixing ability at low temperatures, are difficult to manufacture by the polymerization methods.
In attempting to use polyester resins for non-pulverization methods, JP-A09-34167 discloses one possible method for manufacturing toner. In this method, first, toner compositions including a polyester resin are pulverized into particles, and the particles are then dispersed in an aqueous medium and treated with a solvent, so that spherical toner particles are formed. As another approach, JP-A 11-149180 discloses a method for manufacturing toner using an isocyanate reaction in an aqueous medium. However, neither of these toners have sufficient low-temperature fixability and productivity.